Lil'XT - Ulduar is my favourite raid, just love hearing shout "NO NO NO NO NONONO NO NO!!!" Pretty good finisher when farming lower level pets due to aoe attack. Can self heal to save you a trip to the stable master.

Onyxian Whelpling - One of the first I leveled, very powerful and carried me through a lot of the trainer fights.

Feline Familiar - This thing is a powerhouse. Prowl and Onyx Bite can 1 shot almost any bird, has a shield to block most dots and multi-hits. Can even heal itself with devour.

Chi-Chi - Matches my cloak and really stands out in a crowd. Haven't had much experience fighting with it yet, but it seams useful for setting up Wild Magic and then feign death to block an attack + swap in your next pet.

Rapana Whelk - This guy is a tank. Attack that heals himself, damage reduction shield, and a dive ability to avoid the big hits. I use him against many of the Pandaren Grand Masters.

If I had to pick just five? Juuust five? Well, in no particular order...

1. Flamering Moth. Been doing a lot of pet battle farming lately in the Valley of the Four winds and this moth has become a staple in my farming team. That heal makes this moth worth its weight in gold to me.

2. Cogblade Raptor. A lot of my trainer teams have this guy on them because he's pretty much invaluable against any beast-type pet.

3. Kun-Lai Runt. He's also on a lot of my teams. He hits like a truck.

4. Landro's Lil' XT. For nostalgia's sake, as he's one of the first pets I leveled. He's been useful in PvE as well and I love his emotes.

5. Nexus Whelpling. This guys eats flying pets for breakfast. Also I am a sucker for dragon whelps

I don't have too many pets leveled up yet so these are just kinda off the top of my head.

1) Darkmoon Zeppelin - I just traded off my well worn, trusty zeppelin and immediately had to get another. Decoy has a long CD but it's fantastic if you're leveling up a low level pet and it also to buy some time to whittle down whatever beast you're facing.

2) Nordrassil Wisp (S/S) - I use this in three of my daily Pandaria tamer teams. Flash + Light is very useful against some tamer pets and the S/S outpaces almost everything except flyers.

3) Stunted Shardhorn (P/P) - Before there were direhorns, this was the pet I used to destroy critters. Slow but hits pretty darn hard, especially if you combine the beast racial with Trumpet Strike before using another attack. Even though I now have all the direhorns, I still like how simple the Shardhorn is to use.

4) Yellow Moth (P/P) - When in doubt, this guy is usually pretty good at knocking out whatever you're facing. In general I find moths very user friendly and like almost all their abilities.

1) Frosty - This little scourge of the Frozen Wastes is largely responsible for helping me pick up my Stunted Direhorn just recently. It is a terrific blend of Undead, Dragonkin and Elemental...and would serve just about any comp very well, IMO.

2) Murkablo - He was one of my original 3 lvl 25'ers. Also, very useful against Nairn, the Shademaster's brutal Kun-Lai Runt pet. This Blizzcon badboy could do decently in PvP, I think...but I'm gonna have to do some brainstorming to make it effective.

3) Blighthawk - Even if the opposing pet isn't Humanoid, this zombie of the skies will tear them apart anyway. It's even got a counter to Aquatic pets, and that makes it well worth saving a spot for on a PvP team, IMO.

4) Unborn Val'kyr - I enjoyed this pet before 5.4, and not much has changed about that. I do like that they modified Haunt, even though now it's pretty much an easy-win button 75% of the time. I use it on my Howlbomb team with the Water Spirit and Chrominius to take down most Legendaries, including Chi-Chi.

5) Wolpertinger - Last but certainly not least...this little guy could lock down even the biggest and baddest tanks/maulers pre-5.4. It still fares very well on my Stun Squad, I just gotta be careful about the Resilience buff. Oh, forgot to mention, he's my favorite pet of all-time and one of the very first I was able to get my hands on.

2) White Tickbird Hatchling - the pet I have out on most of my healers when in healing spec. "Healing is under the white bird" I like him because he flies higher than most flying pets, staying out of my way.

Emerald Proto-Whelp because it murders fliers, and pretty much everything else too. Emerald Presence makes it effectively immune to most variants of Stampede and most damage over time effects. Possibly the single most OP pet in the game right now.

Darkmoon Tank because it can kill any beast in one round, which is handy on a lot of trainers. Minefield and Ion Cannon is a devastatingly effective combination. Great for carry teams and useful against legendary beasts too when used with Shock and Awe.

Gilnean Raven because it kills anything aquatic and counters those obnoxious defensive pets. Darkness and burst damage makes this thing cut through both tank and healer pets. The only downside is that it has no defensive options at all.

Pandaren Water Spirit because it has a versatile anti-elemental move set, and trainers have a lot of those. Whether you want a burst pet with Whirlpool and Geyser or a resilient pet with Healing Wave and Dive, this pet can do the job. And it does it without that pesky air vulnerability that aquatic pets normally have (though at the cost of water vulnerability, which is a big handicap sometimes).

my current pvp team that's been winning a lot is val'kyr/monk/xufu. xufu and the monk really dominate with that added dmg from haunt. my opponent almost always has one pet down and my three are almost undamaged. and for pet leveling, lil xt is too good for the early levels, then he still pulls through for the non-panda bag fights, while the emerald proto whelp can almost solo a few panda tamers making him excellent for later leveling.